Oxidation of 8‐Methoxypsoralen (8‐MOP) by Dimethyldioxirane. Evidence for the Formation of a Furocoumarin Epoxide by Methanol Trapping, a Potential Mutagenic Agent

The reaction of 8‐methoxypsoralen ( 1 ) with dimethyldioxirane in methanol at 25 ° C yields the methanol adduct 3 , namely the 2,3‐dihydro‐2‐hydroxy‐3,9‐dimethoxy‐7 H ‐furo[3,2‐ g ][1]benzopyran‐7‐one, whose structure was unequivocally assigned by means of spectroscopic data. Methanol adduct 3 is produced by dioxirane epoxidation of the furan double bond of 1 to 2 followed by nucleophilic attack by methanol at the benzylically activated C‐3. The direct observation of the 8‐methoxypsoralen epoxide ( 2 ) was not possible, because at low temperature (below 0 ° C) dimethyldioxirane does not epoxidize 1 , while at 25 ° C this labile epoxide decomposes. On the basis of these results, a rational explanation is offered as to the origin of the regioisomeric methanol adduct 5 , namely 2,3‐dihydro‐3‐hydroxy‐2,9‐dimethoxy‐7 H ‐furo[3,2‐ g ][1]benzopyran‐7‐one, observed in traces (ca. 0.2%), when 1 is treated with sodium hypochlorite and hydrogen peroxide in aqueous methanol.